1. Field of the Invention
The present invention relates to a multilayer coil and, more particularly, to a multilayer coil which is constructed to be incorporated in high-frequency electronic equipment or the like, and a manufacturing method for making such a multilayer coil.
2. Description of Related Art
An example of a conventional multilayer coil is shown in FIG. 9. A multilayer coil 1 shown in FIG. 9 is formed by laminating a plurality of insulative sheets 3, each of which is provided with a coil conductor 2 on a surface thereof, adding protective sheets 4 and 5 at the top and bottom of the laminate, and then firing the laminate. The respective coil conductors 2 are electrically connected to each other in series through via holes 6 provided in the insulative sheets 3 so as to form a spiral coil 10. Both ends 10a and 10b of the spiral coil 10 are respectively connected to external electrodes 7 and 8 provided at the right and left ends of the multilayer type coil 1 as illustrated in FIG. 10.
Generally, as the inside diameter of the spiral coil 10 is increased, the inductance obtained accordingly increases with increasing Q-value. On the other hand, it is desirable that the size of the multilayer coil 1 is as small as possible. However, as the size of the multilayer coil 1 is decreased, the inside diameter of the incorporated coil is inevitably decreased, resulting in a smaller inductance and Q-value. Therefore, in order to obtain a large inductance and a large Q-value from a compact multilayer type coil, the decrease of the inside diameter of the spiral coil 10 must be minimized. For this reason, attempts have been made in conventional multilayer coil 1 to minimize the gap between the coil conductors 2 and the peripheral edges of the sheets 3.
However, the attempts to minimize the gap between the coil conductors and the peripheral edges of the sheets upon which they are formed have been hindered because of the conventional belief that the gap between the coil conductors 2 and the peripheral edges of the sheets 3 in the conventional devices must be at least about 50 .mu.m for a widthwise gap g1 of the sheets 3 to ensure printing accuracy and insulation reliability of the coil conductors 2. Also, in the conventional coils, a lengthwise gap g2 of the sheets 3 must be usually 100 .mu.m or more in order to prevent deterioration in the characteristics of the multilayer coil 1 caused by the stray capacitance generated between the external electrodes 7 and 8 and the coil conductors 2. Hence, the coil diameter of the multilayer type coil 1 has been significantly smaller in relation to the external size of the multilayer coil 1. As a result, it is difficult to obtain a large inductance and Q-value. Furthermore, the coil conductors of most multilayer coils are shaped such that they are rectangular and have four corners defining right angles. This construction has also contributed to a low Q-value.